When drawing optical fibre, either monomode or multimode, from a silicon glass preform, a cylindrical furnace melts the tip of the preform which is lowered at a controlled rate into the furnace while optical fibre is drawn from the molten tip. The preform passes into the furnace at the top entrance to the furnace and the fibre is drawn through the bottom exit. An inert gas, for example Argon, Nitrogen, Helium etcetera is introduced into the furnace either around the preform entrance or the fibre exit mainly to protect the vulnerable parts of the furnace, which are normally made of graphite. A gas exit port is provided at the opposite end of the furnace to take away the gas. Thus the furnace remains purged with gas during normal operation. The temperature of the furnace is around 2200.degree. C., but could be in the range 1800.degree. C. to 2500.degree. C.
The furnace is surrounded by and insulated from a water-cooled jacket, which also cools the entrance to and exit from the furnace, together with the gas entrance and exit ports.
As the preform melts, a small amount of silica is burnt off as a vapour and carried with the gas and we have found that this burnt off silica tends to condense out on cooler parts of the furnace particularly on the approaches to and near the gas exit port for the purging gas. Eventually this deposit has to be cleaned off otherwise it affects initially the fibre diameter control arrangement and eventually the fibre strength through flaking of the deposit taking place on the fibre being drawn.
In an induction furnace there may be no gas purging through the furnace, but the contamination referred to above also occurs in an induction furnace and, like the graphite furnace, is dependent upon rate of feed of the preform, the furnace temperature, and the surface area of. Natural convection currents will carry the contaminants to the upper entrance.
If fibre is pulled substantially continuously, then apart from replacing the preform, after about twenty hours the contamination referred to has reached a stage at which the fibre specification and strength become suspect, and so the contamination must be cleared out. This requires the furnace to be ramped down and as much as four or five hours production time is lost.
It is an object of the present invention to overcome the difficulties referred to above to enable far better utilisation of the fibre drawing equipment.